Gaseous diffusion process



May 5, 1953 P. w. GARBO GASEOUS DIFFUSION PROCESS Filed May 21, 1946 INVENTOR.

Patented May 5, 1953 UNITED STATES? OFF-ICE.

15 Claims.

This sinventionz'. relatesz xto thetaseparationitof materials in .1 the. :gaseous :state and imore sparticularly to such.separationccarriedout by'imeans of.-;a. diffusion :barrier;

Tremendous impetus ahas a'been-s given r; toxrthe application of. diffusion processes. to commercial 1 operations involving 1 chemicalt'. and/or physical. treatment. of. gaseouszssubstances sbecauseysof the spectacular. success of the 1 diffusion. technique. employed in: producing theatomic bomb... In spite of .the :high state rofzdevelopment reached 1 by-the diffusion process for, the separationrof. uranium isotopes; from: an economic point :of, viewwdiffusion as a chemical engineering tool? or zunitv operationis; still seriously: handicapped by. the slowness of separation.andfiorwwextenw sivenessqof. :ba-rrier ;-area required;to-eifectt=sep+ aratiorrr.

A tprimar-yyy object of :thiswinventionr is :to ;1in,- crease the rate of difiusion processes. AJJCQITOI-l lary obj'ect is to. reduce the barrier area required: for an given 'difiusionprocess-.2. Other-[objects will .beceme apparent from r the description: of the invention which follows;

Inv accordance with; the invention, the: dif:

fusion of: gaseous-'materials through? suitable. porousbarriers or.- membranes is conducted. simultaneously with the r movement of. com minutedesolids over-w the surface of; the -f-b3Jw-r= riers. orfmembraneszz. It. is .-generally-advisable; to use the solids in as fine a stateot:subdivision: as .is practical .forathe .particularfapparatus; em-

ployed. Obviously, veryfine .powders which tendi to .jclog.gtha-capillaries-. of the diffusion-abarrier r shouldrbe avoided solids should have a particle -r size .-'-'somewhat-; larger than. .the'ad-iameter of theecapillaries in the" barrier with which the. solids .-.-wil1. rhea-used. Generally, speaking; the T desirable particle sizes will ube" found in :the. sieve size: range of. about 60-to.325.mesh, preferably aboutlOOto 200. ;mesh;

partic'le'shape as will 'avoid or minimize: abrasion" of the: diffusion barrier.

The movement of) the-- comminuted solids over: a diffusion barrier can be achieved in many different ways: Thus; one simple method is to placexthe"diffusion-barrier steeply inclined "to'the In '-short,z thecomminuted iii) horizontal: plane-and'idroptpowdered material .011!

theibarrier::neanritsrupper end so -that the ipowderzziwill-nlflow over :the;:barrier .to its lower end. 1 Anothergmethod involves disposing, the diffusion barrienfincontact with amass of powdered ma terial and then shaking or. otherwise vibrating the ."apparatus to: cause. movement of the powder w ardly through-la amass of powdered material with:sucha.velocity was to suspend, andtyet permit slippage -orq hindered settling of the powder inthe. gaseous stream. Under these con-.- ditions; the. :individual particles of. the mass ..de.-.-

scribe ,randommovements and. the fluidized. mass,

assumes-.atheturbulentappearance of .a boiling liquid.-.

To .deseribe-eandi clarify-theprinciples of my: invention inugreaterxdetail, reference is made to :the accompanaying drawings which. are schematic representations.,.of typieal forms. of rapparatus suitablea for. carrying out. improved:

gaseous diffusion processes.

Figured is .arsectionallelevation.of a gas diffusion. cell. wherein, fluidized powder-is, maintained inqcontact with, both faces-eitheporous barrier;

Figureg isa.sectionaLelevation of another cell.

wherein the diffusion barrierfis in theformof hollow cones; immersedeinv a fluidized mass of powder;

Figure-13-.is;a,.,sectional side viewof. a slopingcell .whereimcomminuted .solids charged... at the upper-:end. slide-gacross thetop surface of the.

barriergto the-lower end of the cell; and,

Figure. 4 is a sectional elevation of a..cell. hav-v ingpowderedmaterialon both sides of theporous membrane and. mechanical means for moving the cellup. and-down and. thus ,oausingthe powdered.

material to. be .shaken within the .cell..

In Figure 1,. the diffusion cell I may becircular, rectangular or of anvdesired shape in horizontal crossrsection. If] the horizontal section of the. celliscircular, thaporous barrier2 is a .tubeoff smaller diameter .than that of the cell so that a central zone .3 and an annular zone l are formed within cell l tal.-;crosssection; then :a pairof-barrier walls 2, disposed. parallel: .to; each/ether, divide :the cell intosagcentral Z01'18T3 and two outer zones l along opposite. sidesxofl the cell... The gaseous mixture I which 'fisfto. he-.-separa ted or enriched enters cen- If-ce1l II is rectangular in horizon? tral zone 3 through pipe and leaves through pipe 1. A purge gas to sweep out of zone 4 the gas which has diffused through barrier 2 enters through pipe 6 and leaves together with the diffused gas through pipe 8. As thus far described, cell I might be said to be representative of cells adapted for performing conventional diffusion processes. However, in accordance with the invention, zone 3 has a fluidized mass of comminuted solids with a pseudo-liquid level 3a, while zone 4 holds a similar mass with pseudo-liquid level 4a. The gaseous mixture entering by way of pipe 5 passes up through zone 3 at such a velocity that th powder therein is fluidized. In the form of powders of about 100 to ZOO-mesh particle size, most of the common materials encountered, such as clay, silica, copper and magnetite, are usually fluidized at gas velocities in the range of about 0.1 to about 5 feet per second, preferably about 0.5 to about 2.0 feet per second. The original gas entering through pipe 5 less the portion which has diffused through the barrier 2 into zone 4 becomes disengaged from the bulk of the powdered mass in zone 3 at the pseudoliquid level 3a, passes up through gas space 9 and is discharged through pipe 1'. Any entrained solids which do not drop out of the gas in space 9 are carried out through pipe 1 by the gas to a cyclone, electrostatic precipitator or othe separator for removing fine solids from gases. solids so recovered are returned to zone 3 in any desired manner, e. g., by suspension in the gaseous stream entering through pipe 5.

While the fluidized mass of powder in zone 3 increases the rate of gaseous diffusion through the porous membrane 4 over that observed in the absence of the moving particles, a further increase is made possible by fluidizing powder in zone 4 in contact with the other face of the membrane 4. Thus, the purge gas entering through pipe 6 flows up through zone 4 fluidizing the powder and sweeping out the gas which has permeated through diffusion barrier 4. The combined gases become disengaged from the bulk of the powder at pseudo-liquid level'4a, pass through gas space I!) and leave through pipe 8. Entrained solids may be handled in the same manner as was discussed for the operation of zone 3. The mixture of purge gas and diffused gas is then separated by conventional methods,

e. g., condensation of one component, or otherwise treated as desired.

Figure 2 shows a vessel or cell H with a plurality of hollow conical barriers l2 suspended therein. Around the conical barriers [2 there is a fluidized mass of comminuted solids [8 with a pseudo-liquid level IS. The gaseous mixture which is to be subjected to separation by diffusion enters through pipe I! and fluidizes the powdered mass I8. Part of th gaseous mixture diffuses into the hollow cones l2 from which it is withdrawn by way of domes l5 and manifold pipe It. To facilitate the prompt removal of the diffused gas from the hollow cones l2, it is advantageous to feed a purge gas into manifold pipe 4 from which it discharges into cones l2 by way of internal tubes 13. The purge gas thus sweeps out the diffused gas by flowing up through domes l5 and manifold 16. The residual gas in the cell emerging from the pseudo-liquid level l9, flows through gas space and passes through a filter 2|, e. g., a porous alundum tube, which prevents the escape of powder by entrainment in the gas. The residual gas freed of powder then discharges from the cell through pipe 22.

The

It is well to note that the conical shap of the barrier elements l2 serves to compensate for changes in the volumes of the gases along the height of the cell and, accordingly, to maintain substantially uniform gas velocities up through the cell. Thus, as the gas mixture flows up through the fluidized mass [8 its volume decreases steadily because part of the gas diffuses into hollow cones l2. At the same time, the gas in each cone l2 increases in volume as the gas moves up from the apex to the discharge dome !5. The conical shape of the barrier elements [2 compensates for these volume changes by providing in a direction upward through the cell an increasing cross-sectional area within the barriers and a decreasing cross-sectional area outside of the barriers.

An advantageous modification of the diffusion system just described is to fill the hollow cones [2 with finely divided solids which are then kept in a fluidized state by the purge gas fed into the cones by internal tubes [3. The fluidized solids within the cones tend to increase further the rate of gaseous diffusion through the porous membranes beyond the improved rate attained by the fluidization of solids in contact with only the exterior surfaces of the membranes.

Figure 3 schematically presents a diffusion cell with a sloping barrier partition 26 forming zones 21 and 28 therein. The gaseous mixture to be processed in the cell enters through pipe 29, flows up along barrier 26 through zone 21 and leaves through pipe 30. Some of the gas in zone 2'! diffuses through barrier 26 into zone 28. Purge gas fed through pipe 3| assists in withdrawing the diffused gas from zone 28 by way of pipe 32. In accordance with the invention, the rate of gaseous diffusion through the porous partition 26 is increased by causing comminuted solids to slide down across the upper surface of partition 26. For this purpose, solids placed in hopper 34 are charged through rotary bucket-type valve 35 and pipe 36 into zone 2?. The resulting moving layer 33 of solids on partition 26 gravitates to pipe 31. Another rotary valve 38, operating at the same rate as valve 35, discharges the solids into hopper 39. Conveyor and elevator means (not shown) return the solids from hopper 39 to hopper 34 to complete the cyclic movement and thus ensure continuous circulation of solids over the porous barrier 26.

In Figure 4, the diffusion cell 40 is partitioned by membrane 4| into zones 42 and 43 containing masses of finely divided solids with top surfaces 42a and 43a, respectively. The gaseous mixture to be enriched by diffusion flows through flexible hose 44 into zone 42 and leaves through hose 45. Part of the gas in zone 42 diffuses through barrier 4| into zone 43. Purge gas entering through hose 46 facilitates the removal of diffused gas from zone 43 by way of hose 41. The base member 48 of cell 40 is provided with a bearing 49 with which is associated a connecting rod 50. The other end of rod 50 is eccentrically connected to a rotating element 5|. Cell 40 is mounted between upright guides (not shown) which permit the cell to slide up or down. The rotation of element 5| imparts through rod 50 a vertical reciprocating motion to cell 40. The frequency and amplitude of reciprocating motion are adjusted so as to shake the powder in zones 42 and 43 and maintain it in good rubbing contact with the faces of barrier 41 substantially continuously. Operating in this manner, the solids are kept in a fluent and agitated state resembling the state attained by the racemes :iluidizationtechnique. "However;smechanicaliagi- :tationwas exemplified byrthe apparatus. ofFigure 4 permits operation voiflthe :diiiusionclhto achieve the purposes of the invention under conditions whichare not conducive to effective fluidization, for example, passing the gases lthroughthe cell at very low velocities.

' It is clear thatthe principles of this invention are applicable -'to 'any process involving =Ith'e diffusion'o'f gas through a porous barrier 'or membrane without any restrictions relative to" the gases separated or enriched, the type of "barrier ernployed, the pressure in the cell, etcfi'I'hose skilled in the art of gaseous diffusion willappreoiate that operating aconditionsin any diifusionprocess; such as thesize of the pores or capillaries .in'the :barrier, "the gaspressure inthe cell and the "rate of passage 'of'gas "through the cell, should :be carefully controlled in order to obtaina ;high "separation 'efficiency. The work .offiliorenz and Magnus, reportedzin Z. anorg. allgem. Chem. 1316-,

97-133 1(1924), is representative of the available technical literature :on gaseous diffusion operations.

The barriers selectedior the cells maybe made 1 of metals or nonmetallic materials depending uponlthe service conditions to Whichthey will be exposed. Since the fabrication"of'difiusion barriers .is well established and does :not constitute a part :of this invention, details of barrier-preparation need "not be presented herein.

.Ithas been stated hereinbefore that the oom- :minuted solids used in-any given-I difiusion system should be ofsuch composition as not toadversely influence or react with the gasesinthe diffusion '5 cell. This is not to "saythat only inertsolids may be used and that solids-'whiehlinteract wvithlthe gases in a beneficial 'or-desirable way are precluded. To the contrary, itiszpreferred to :select solids whichqfunctionrnot(only to improve the rate "of' gaseous .diiiusi'on by their movements along the surface of the porous membrane but also to bring about .an independentusefulresult. Thus, for instance, a two-componentgaseous mixture containing .a-small proportion of impurity, :e. g., hydrogen sulfide, may .be subjected to separationby gaseous diffusion while maintaining in contactwith the porous :barrien a .moving mass .of a powdered solid capable of taking up the :gaseous impuritytbygphysical ad- :sorption or chemical-reaction. In such case, the

powder would. .be'periodically or-continuously withdrawn :from the difiusionrcelland replaced by fresh or regenerated .powder in order to prevent the powderin the cellr'from becoming completely spent and thus inoperative in respect .to the continuedlclean-upof impu-rityin thewgaseous mixture. Another type of "dual-purpose comminuted solid is one that catalyzes adesired'reaction in a gaseous mixture which is undergoing separation .or enrichment by diffusion. To .illus- .trate this form of the invention,:reference= willbe .made i to the classic Water-,gasshift' reaction:

The-gaseous eifiuent'from aconventional'watergas generator comprising carbon monoxide-and dioxidahydrogen and water vapor is passed at an elevated temperature into a diffusion can,

such as that of Figure l, having a barrier-which permits the preferential diffusion "of hydrogen.

The cell (holds a mass of powdered iron oxide catalyst which is fluidized bytthe incoming gases.

Umecontact ofithe gases with the'powdered iron oxide at an elevated temperature, sayabouttuo" diffusion process.

I of the operation. to require accuratetcmperature regulation since F.,rcatalyzes;the.i'oregoing reaction in the :direeztion of producing. hydrogen-which for many purposes is' the desired component. While the gases arelfiowingzthroughrthe fiuidizedrmass of-catalyst in: the "diffusion cell, some of the :originai'hydrogen as well as newly formedhydrogen diffuses through. the :porous barrier at a higher rate than 'wouldobtainin the absence of'thefluidized mass.

This'diffusi'on o'fhydrogen has two important advantages. Fiist, the diffused hydrogen is readily recoverable ."as *an enriched stream, and

is'e'condthe loss of hydrogen by difiusion irom'the :mixture in contactn'with the fluidized catalyst favors the production of vadditiona1 hydrogen-in accordance with the physicmchemical law of mass action. Accordingly, the moving particles 10f iron oxide catalyst function simultaneously to :increase theLrateIof gaseous difiusi'oh through the porous membrane and to promote the production of hydrogen. Numerous analogous chemical reactions of gases or vapors which wouldbedoubly benefited by the foregoing form of theinvention "will occur to those skilled in :therart.

:It isiindeed surprising that the simple expedient of maintaining moving particles in good l-rubbing contact with .0116 or both sides of a porous barrier can improve the diffusion process.

While-my invention is not to'be limited by any theory of operation, I have postulated a-inechanism that may account for the unexpected results. I visualize that the usual operationfi'of a diiliusion cell leads to the formation of slow ll'l0l'ing fi111'iS of gas 011 1301311 facescf the'porous -barrier with consequent retardation of the diffusionzprooessbutthat the use of moving particles in contact with the barrier tends to disrupt or minimize the film and thus facilitates the unobstructed flow of gas by "diffusion through the barrier. Furthermore, where fiuidization is employed to maintain the moving particles'in rubbing relation with the barrier, I imagine-that another useful influence "may be exerted on the It would appear that as :a stream of a "gaseous mixture flows along "the surface of a porous membrane the layers of the stream closest to the membrane become deficient tinithedifiused-component and therate of diffu- :.sion is slowed down by this impoverishment" until the same component flowsirom the farthest layers of thestreamtothose .cl-osestto thesmembrane. In fiuidization, the. gaseous mixture is constantly in turbulent motion so that theportion of the-stream closest to the barrier at :any

level in :theiiuidized mass is atall times substantially the same incomposition as the-portion iarthestfrom the barrier. Inshort, fiuidization gives :rapid, continuous mixing which prevents stratification of the gaseous-mixture flowing up :along the surface :of the'barrier.

The presence of a body of moving particles,

particularly fluidized particles, in contact with a membrane for gaseous diffusion pliers. anopportunity for closely controlling the temperature Difi-usionprocesses areiknown temperature variations lead to .pressure and velocity fluctuations, eddy currcntsrin the gaseous stream flowing through the diffusion cc and other effects whichimpair operational efficiency. A. body ref moving particles withinithe diifusion cell acts as a thermal flywheel-stabilizing and equalizing the temperature throughout the cell. .In addition, heat transfer surfaces, oi as tubes or coils, disposed in the ccll and surrounded by them-asset moving powdermay beused for close temperature control. In the absence of the powder such heat transfer surfaces would set up disturbing eddy currents in the gaseous stream. The combination of heat transfer tubes or coils and fluidized solids in a diffusion cell is especially advantageous where the mixture of gases flowing therethrough is at the same time undergoing chemical reaction. In such instance, heat is readily added or withdrawn, respectively, for an endothermic or exothermic reaction by circulating a suitable fluid through the tubes or coils. The fluidized solids assure a rapid and even transfer of heat between the gaseous stream and the tube or coil surfaces.

Those skilled in the art will visualize many other modifications and variations of the invention set forth hereinab-ove without departing from its spirit and scope. Accordingly, the claims should not be interpreted in any restrictive sense other than that imposed by the limitations recited within the claims.

What I claim is:

1. The improved gaseous diffusion process for the separation of mixed gases, which comprises passing a gaseous mixture along one surface of -a diffusion barrier adapted to permit the prefercomminuted solids under fluidizing conditions, I

the thus fluidized mass being in contact with a diffusion barrier adapted to permit the preferential diffusion therethrough of one component of said gaseous mixture, and recovering the gaseous diffusate.

3. The improved gaseous diffusion process for the separation of mixed gases, which comprises passing a gaseous mixture along one surface of a diifusicn barrier adapted to permit the preferential diffusion therethrough of one component of said gaseous mixture, passing a purge gas along the other surface of said barrier, while maintaining moving solid particles in contact with at least one of said surfaces of said barrier, and recovering the gaseous diffusate together withsaid purge gas.

4. The improved gaseous diffusion process for the separation of mixed gases, which comprises passing a gaseous mixture along one surface of a diffusion barrier adapted to permit the preferential diffusion therethrough of one component of said gaseous mixture, passing a purge gas along the other surface of said barrier, while maintaining a mass of fluidized solid particles in contact with at least one of said surfaces, and recovering the gaseous diffusate together with said purge gas.

5. The improved gaseous diffusion process for the separation of mixed gases, which comprises simultaneously passing a gaseous mixture under fluidizing conditions through a mass of powder in contact with one surface of a diffusion barrier adapted to permit the preferential diffusion therethrough of one component of said gaseous mixture, passing a purge gas under fluidizing conditions through a mass of powder in contact with the other surface of said barrier, and withdrawing separate gaseous streams from the two said masses of fluidized powder.

6. The process of claim wherein the gaseous mixture comprises two principal components and one minor component and the fluidized powder in contact with the first said surface of the diffusion barrier is capable of reacting with a component of said gaseous mixture .to remove it from said gaseous mixture.

'7. The process of claim 5 wherein the gaseous mixture comprises two principal components and one minor component and the fluidized powder in contact with the first said surface of the diffusion barrier is capable of adsorbing a component of said gaseous mixture to remove it from said gaesous mixture.

8. The improved process for conducting catalytic reactions of gaseous reactants, which comprises passing the gaseous reactants at reaction conditions through a mass of powdered catalyst at a fluidizing velocity, maintaining the thus fluidized catalyst mass in contact with a diffusion barrier adapted to permit the preferential diffusion therethrough of one gaseous product of reaction, withdrawing reaction gases from said fluidized catalyst mass, and recovering the gas diffusing through said barrier.

9. The improved process for conducting catalytic reactions of gaseous reactants, which comprises passing the gaseous reactants at reaction conditions through a mass of powdered catalyst at a fiuidizing velocity, maintaining the thus fluidized catalyst mass in contact with one side of a diffusion barrier adapted to permit the preferential diffusion therethrough of one gaseous product of reaction, passing a purge gas at a fluidizing velocity through a mass of powder in contact with the other side of said barrier, and withdrawing separate gaseous streams from the two said fluidized masses.

10. In gaseous diffusion processes involving the use of a diffusion barrier for the separation of mixed gases, the improvement which comprises maintaining a substantially continuous layer of moving, finely divided, solid particles in contact with the diffusion barrier.

11. In gaseous diffusion processes involving the use of a diffusion barrier for the separation of mixed gases, the improvement which comprises maintaining a mass of fluidized solid particles in contact with the diffusion barrier.

12. The improved gaseous diffusion process for the separation of mixed gases, which comprises simultaneously passing a gaseous mixture through an agitated mass of powder in contact with one surface of a diffusion barrier adapted to permit the preferential diffusion therethrough of one component of said gaseous mixture, passing a purge gas through an agitated mass of powder in contact with the other surface of sadi barrier, and withdrawing separate gaseous streams from the two said agitated masses of powder.

13. The process of claim 12 wherein the gaseous mixture comprises two principal components and one minor component and the agitated mass of powder in contact with the first said surface of the diffusion barrier is capable of combining with said minor component of said gaseous mixture.

14. The improved process for conducting catalytic reactions of gaseous reactants, which comprises passing the gaseous reactants at reaction conditions through an agitated mass of powdered catalyst, maintaining said agitated mass of catalyst in contact with one side of a diffusion barrier adapted to permit the preferential diffusion therethrough of one gaesous product of reaction, passing a purge gas along the other side of said barrier, and withdrawing separate gaseous streams from the two opposite sides of said barrier.

15. The process of claim 14 wherein the purge gas passes through an agitated mass of powder in contact with said other side of said barrier.

PAUL W. GARBO.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,124,347 Smelling Jan. 12, 1915 1,174,631 Snelling Mar. 7, 1916 1,496,757 Lewis et a1. June 3, 1924 Number Number Name Date Hale et a1 Mar. 13, 1934 Odell Dec. 18, 1934 Frolich Dec. 1, 1936 Maier Sept. 9, 1941 Huppke Dec. 5, 1944 Bates July 31, 1945 Stahly Oct. 30, 1945 Gohr et a1 Oct. 28, 1947 Brandt Nov. 25, 1947 FOREIGN PATENTS Country Date Great Britain May 4. 1923 

1. THE IMPROVED GASEOUS DIFFUSION PROCESS FOR THE SEPARATION OF MIXED GASES, WHICH COMPRISES PASSING A GASEOUS MIXTURE ALONG ONE SURFACE OF A DIFFUSION BARRIER ADAPTED TO PERMIT THE PREFERENTIAL DIFFUSION THERETHROUGH OF ONE COMPONENT OF SAID GASEOUS MIXTURE, WHILE MAINTAINING MOVING SOLID PARTICLES IN CONTACT WITH SAID SURFACE OF SAID BARRIER, AND RECOVERING THE GASEOUS DIFFUSATE. 